Power ceramic hybrids have been widely used in aerospace systems for power control, power distribution, and power conversion. Because of the customized nature and material requirements associated with typically hermetically sealed power hybrid packages, power hybrids for electronics systems are high in cost and difficult to obtain. Further, in recent years, hermetically sealed components have become difficult to find, and a move has been made to commercial grade Plastic Encapsulated Microcircuits (PEMs).
However, there are long-term reliability problems associated with commercial PEM devices. PEM packaging often suffers from failures caused by contaminants embedded in epoxy molding compound, and by absorption of moisture.
Contaminants could severely degrade PEM devices' parametric performance. Moisture can penetrate through the PEM package and lead to corrosion, loss of adhesion of the epoxy and increased leakage current. Simulation models for predicting lifetime of PEMs lack validation data, and are therefore, questionable. On the other hand, subjecting parts through “test-to-failure” programs can be very costly and time consuming.
Further, Commercial Off-the-Shelf (COTS)/PEM devices are not in full compliance with certain requirements and not recommended for use for high reliability applications in severe environments.
Alternatively, to provide the appropriate hermeticity and power requirements for a reasonable cost, low-cost printed circuit boards have been widely used as substrates for direct mounting of semiconductors in commercial electronic products. This approach is often referred to as Chip-on-Board (COB) or MCM-L (multi-chip module-laminate) packaging technique. However, the COB approach has been typically used for circuit applications at low power levels.
In particular, conventional COB electronic assemblies often use a glob-top material for device encapsulation. However, a glob-top does not provide the hermeticity and power required, or good mechanical protection against damage from foreign objects, and is relatively ineffective as a seal against moisture and contaminants in severe environments.
Further, in power electronics applications, the electronic components are constantly under severe electrical and thermal stress due to high transient voltages and high transient current, with fast rise and fall switching times present in the circuit. Sufficient heat removal for keeping device junction temperatures below their maximum allowable limits becomes a major issue in power hybrid design.
To prevent potential ground current loops, electrical isolation is required between the device terminal connections and the heat sink on which the device is mounted. Depending on specific system applications, electrical isolation requirements could reach from several hundred to several thousand volts.
Accordingly, low cost Commercial Off-the-Shelf (COTS) electronics components, such as commercial near-hermetic COB packaging or a Power-Chip-On-Board (P-COB) technology for high power levels, as replacements for high cost, hermetically sealed electronics parts for, for example, military applications, are needed to meet avionics requirements and reduce component costs.